1. Field of Invention
The present invention relates to dielectric well logging techniques.
2. Description of Prior Art
In dielectric well logging operations, a magnetic field was generated in a transmitter coil, which was lowered in the well, to cause conduction and displacement current flow in a formation surrounding a well bore. The displacement current flow induced a voltage in a receiver coil spaced from the transmitter.
However, due to the presence of fluids such as drilling mud, the well bore was somewhat conductive, causing a signal coupling path through the mud column in the well bore. The unwanted signal travelling along this path was the result of an electric field effect and was often called an electrostatic or capacitive signal. U.S. Pat. No. 3,094,658 describes structure directed to an effort to remove the effect of this unwanted capacitative signal during induction well logging at frequencies, such as at approximately 20 kilohertz. U.S. Pat. No. 3,893,021, of which Applicant is a co-inventor, discloses another dielectric well logging system in which electrostatic shields of this general type are used.
However, when the frequency of the system was raised into the radio frequency spectrum, problems with prior art shielding arose. Logs obtained in the same formation differed when different size outside cases were used for the logging system.